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Video Borescope

by Peter G. Lorenz*

 

Visual testing used to be so simple: you looked at the specimen! Then came all sorts of optical aids, illumination improvement and photographic archiving as well as standards. I remember marveling as I looked inside the cylinders of my 1968 Mustang without removing the head. Now there are improvements that have passed me by. Thanks to this month's author for an excellent update, explanation and recommendations on video borescoping. How nice! As the problems get tougher, our tools get better.


Frank Iddings
Tutorial Projects Editor

 


Figure 1-2
Table 1

INTRODUCTION

Visual testing and remote visual testing instruments are rapidly evolving with advances in video technology. Beginning with "eyeball" tests using optical borescopes or flexible articulating fiberoptic fiber borescopes (endoscopes) to view internal discontinuities of machinery, video borescoping began when inspectors first mounted boxy analog video tube cameras to the borescope eyepiece and observed tests on a television. As off the shelf video cameras became smaller with charge coupled device (CCD) chips, it became common for most internal visual tests to be viewed on video monitors. Today, coupling external cameras to optical borescope eyepieces, though still done, is becoming infrequent because of another new development: ever smaller imaging CCD chips replacing borescope optics or flexible fiberoptic image guides.

 

Video Borescopes
A flexible video borescope insertion tube, with an articulating tip, houses a small CCD within its tip diameter (Figure 1). As a portable complete visual testing system, its components are built into conveniently transportable cases - the scope, the camera control unit, digital image processor or computer, its light source and system video display are all wired internally for quick assembly and simpler, more effective test procedures (Figure 2).

Increased video borescope usage is due mainly to nondestructive testing professionals.

Increased video borescope usage is due mainly to nondestructive testing professionals. They are first shown new scope technologies and suggest refinements. It was their voiced objections to the many wires, bulky video components and setup complexity as well as their need to measure discontinuity size that led to the latest evolution - the single portable case integrated video borescope system with digital enhancements.

 

VIDEO BORESCOPE EVOLUTION
The following represents the evolution of video borescope digital features and system case technologies.

  • Mid 1980s: 10 mm (0.4 in.) diameter tip articulating industrial CCD video borescope, black and white chip with red, green and blue color sequencing.
  • 1985: 38 mm (1.5 in.) diameter nonarticulating (pipe scope), full color CCD industrial video borescope.
  • 1986: first portable system case video system - the internal dimensional analysis kit. Built in color monitor with compatible keyboard; image freeze and video discontinuity measurement; digital video image phone linkage to transmit test images stored on floppy disk.
  • 1990: 11 mm (0.43 in.) diameter, articulating, full color CCD video borescope. Second generation portable system case, built in cathode ray tube, keyboard, analog to digital conversion, internal 100 MB hard drive or floppy disk; image comparison, internal modem image transmission, video cursor measurement, menu driven image processing, floppy disk loadable upgrade software and case with handle.
  • 1991: first battery powered, over the shoulder portable, CCD video borescope system. Shadow reference line projection measurement, joystick activated motorized cable pull versus hand knob operated scope tip articulation; 8 mm (0.3 in.) diameter, articulating, full color CCD video borescope; third generation portable system case, built in miniature computer, digital zoom, miniature cathode ray tube, light source, camera control unit, three dimensional computer aided design wire frame measurement of stored digital images, case and handle; remote visual testing report software, text template, digital images and digital voice annotated inspector comments.
  • 1992: video borescope curvilinear pipe wall pit measurement using pipe axis alignment correction; three dimensional computed aided design wire frame discontinuity measurement by video borescope.
  • 1993: fourth generation portable case system with built in or remote liquid crystal display, scope camera control unit, light source, memory card and SCSI port for external digital accessory expansion options; optical borescope eyepiece discontinuity measurement using digital processor with interchangeable off the shelf CCD eyepiece camera, developed and patented in the US.
  • 1995: 6 mm (0.24 in.) diameter, articulating, full color CCD video borescope.
  • 1996: live transcontinental US Air Force F100 engine test via digital image exchange between Savannah, Georgia, and Berlin, Germany.
  • 1997: digital store measure industrial miniature computer for scope systems; unique architecture utilizing miniature ball grid array digital signal processing to meet stringent small size specifications; simultaneous live digitized eddy current impedance plane read out on live video image; video borescope with stereoscopic measurement of discontinuity size using dual objective base line separation calculations, scope handle thumb mouse for menu navigation, internal microphone for annotation of stored images and compatible miniature liquid crystal display; hands free goggles to view tests.
  • 1999: 5 mm (0.2 in.) diameter, articulating, full color CCD video borescope.
  • 2000: smallest system case to date; brightest internal light source; integral system liquid crystal display in scope handle.
  • 2002: fifth generation system case video borescope with live digital motion capture; USB fast digital communications port; removable plug in image storage media; personal computer programmable and activated scope articulation; depth perception and three dimensional viewing of test object's internals; 4 mm (0.16 in.) diameter, articulating, full color CCD video borescope.

 

Color
Only black and white CCDs in the mid 1980s were small enough to design a practical 11 mm (0.43 in.) or less diameter articulating industrial video borescope. At that time, 11 and 8 mm (0.43 and 0.3 in.) optical fiber borescopes dominated the market and inspectors had already learned to prefer color test images from cameras connected to their eyepiece. Credit goes to Welch Allyn for an ingenious solution - the red, green and blue color filter wheel. This industrial scope used a small black and white CCD and illuminated objects sequentially under red, green and blue to produce color. Full color CCDs were first used in larger diameter, nonarticulating industrial video borescopes. As the full color CCDs became smaller, their better color rendition and minimal mechanical processing made them the video borescope CCD of choice (Table 1).

 

Video or Optical?
Video instrument tests are increasingly preferred over optical instrument tests because video allows the inspector to see more area and more distant or higher magnified detail, as opposed to a limited, cropped, darker traditional borescope optical image. They also help the inspector to avoid eye strain and test fatigue by more comfortably viewing a display with both eyes and allow inspectors to see higher resolution and avoid moiré interference, the mosaic dots of a flexible image guide fiber bundle. In addition, video testing allows inspectors to conveniently record test results, enhance details with digital processing, perform immediate image analysis and perform multispectrum imaging (video can display wavelengths invisible to the eye).

Original equipment manufacturers design integrated video component test systems to perform a combination of the following:

  • achieve maximum system compactness, portability, setup and operating simplicity
  • incorporate multiple function features to enable tests under many conditions
  • more easily perform one test application or design a best fit specific to equipment geometries
  • offer exclusive performance advantages at a strategic price.

 

COMPONENTS
Inspectors should consider the following component factors when selecting and using a video borescope.

 

Video Borescope Charge Coupled Device
Total pixel count is important, but is not the only or even the most critical performance factor. Same diameter scopes may have different CCD sizes. Though total pixel count is a factor in performance, those with fewer pixels per overall CCD area have larger individual pixels which sense more light per individual pixel. This, in turn, generates a higher signal voltage per pixel allowing more discrete camera control unit processing, similar to the benefits of high audio wattage enabling greater sound fidelity. Video borescope type CCD sensors are supplied by only a few manufacturers, therefore, multiple scope models and brands may use the same CCD. The miniature CCD package circuitry and delicate fixed length umbilical signal wires (the length of the scope's tube) usually cannot be repaired or salvaged because of its costly, complex production. Don't rely on pixel count only in judging performance: test CCD performance yourself by testing the scope system's displayed video image using a resolution target (United States Air Force, 1951).

 

Light Guide
A flexible fiber bundle (glass, quartz or plastic fibers) or one liquid filled tube, directs light from the light source lamp focal point to the scope distal end. Each type has different wavelength performance characteristics transmitting white light, ultraviolet for dye penetrant luminescence or infrared scope applications.

 

Light Source
A light source is usually integrated within a video borescope's main body, producing white light. The mechanics of light guides and lamp fixturing usually do not allow utilizing a secondary external light source of different performance (that is, brighter, ultraviolet); maximum brightness depends on well focused lamp or arc energy into the light guide, not only total wattage (that is, a reflector which focuses 100% of 40 W versus only 20% of 100 W).

 

Distal Illumination in Place of Fiber Light Guide
Usually larger diameter, long length video borescopes (pipe cameras) illuminate the test area by housing miniature lamps or light emitting diodes within their distal end.

 

Lenses for Charge Coupled Device and Light Guide
A compound lens in front of the CCD performs the critical functions of maximizing image resolution and setting the field of view. A lens in front of the light guide helps distribute illumination evenly. All lenses are not equal. High quality endoscope lenses must avoid imperfections, misalignment, coating voids or trapped microscopic dust during assembly which causes astigmatism (only the radial lines of a test target are in sharp focus, not tangential lines), curvature (the center and edges of the field of view are not in focus together) and distortion (if a grid line target is viewed, the edge lines appear curved). In addition, scope resolution will degrade with microscopic lens shifting from wear and tear and transportation shocks. Before every test, check performance using a resolution target (United States Air Force, 1951).

 

Camera Control Unit
Usually housed in the system's main body, the camera control unit's most basic function is to process the encoded image signal from the CCD for display. Today, auto exposure, adjustment of color saturation, contrast, gain and white balance are a relatively standard set of functions. As the video borescope offers more features, the camera control unit requires more memory or the processing power of a board mounted computer. Image freeze, extended exposure, edge enhancement, digital zoom, panning, image inversion and split screen comparison are expected basic video borescope system features. More enhanced features include onboard image and text archiving functions, video overlay options, noncontact discontinuity measurement, digital audio or live motion recording (streaming), external personal computer compatibility and real time digital image conversion or correction. This progression of complexity within a camera control unit could cause some image signal loss. What is critical for inspectors is to apply informed judgment about when features could confuse the test result and when their special functions can help find the discontinuity.

 

Video Borescope Display
Today's integrated video borescope systems have a portable system liquid crystal display but are also compatible with larger, higher resolution, stand alone accessory displays or "hands free" face mounted displays. Video display original equipment manufacturers are also a small group and their huge markets, not the comparatively small video borescope volumes, drive the specifications of these displays, that is, consumer market personal video viewers. Video display resolution is described in terms of scan lines - horizontal and vertical plus overall pixel count based on the early IBM video graphic array standard. Quarter video graphic arrays have a 320 by 240 (overall 76 800 pixel) resolution; full video graphic array is 640 x 480 = 307 000 pixel resolution; super video graphic array for larger size liquid crystal displays is 800 x 600 = 480 000. The inspector's judgment is needed to determine which display resolution maximizes the test results. Which display resolutions, in combination with the scope CCD resolution, will resolve the critical discontinuity size to meet the test standards?

The industrial optical borescope still in use, and used for many years, established application specific visual testing and remote visual testing resolution standards. The more recent video borescopes do not yet have this long standards history. The variables which determine video borescope and companion video display resolution (and digital documentation resolution) are so numerous, a brief overview can't describe them all. Establish your own equipment's resolution for each video borescope test and system configuration. The resolution value is observed when viewing the test target at the test site from a constant fixed distance.

In summary, every time you use a video borescope, be aware of its capabilities and limitations. Inspectors, please test them yourself - repeatedly. Every major component of the video borescope system has features to complement the system design but each component can also contribute limitations.

 

THE FUTURE
Video borescopes will expand well beyond their traditional role in the maintenance and quality control testing of engines, turbines and pipes. For example, a hands free portable video borescope for tactical military or police surveillance can be mounted to bulletproof vests and display visible or invisible infrared images. Portability technologies and wireless connectivity (such as wireless fidelity Internet access, an inexpensive local area radio signal) will establish many more industrial video borescope uses.

 

Improvements and Standards
Scope technologies to accomplish internal tests will always be needed: steerable self propelling housings, miniature zoom or dual optic lensing, less costly CCD or CMOS chips, smaller and brighter light emitting diode illumination and nonmechanical tip articulation. Also needed is technology to provide digital overlays of eddy current readings, infrared indications or ultraviolet fluorescence onto video, live three dimensional internal testing scene orientation of scope position within the work piece, multiple combinations of internal visual plus radiation count or chemical constituent data at the distal tip sensors, programmable pixel saturation values for high speed turbine blade count and automatic onsite trend analysis of blade comparisons every time a plane lands. Video borescope system cases and high costs will likely disappear. It is not too improbable to imagine a cellular phone sized system as digital option packaging becomes available.

Beyond image enhancing features, digital processing has primarily driven noncontact discontinuity measurement as the most desirable feature of video borescope systems. However, quantitative remote video tests cannot progress as fast as this powerful technology could by utilizing lasers, until more accuracy and repeatability standards (obtained by different inspectors using the same equipment) are established by original equipment manufacturers and industry review bodies. Each video borescope user should work towards the establishment of remote visual testing standards within his or her organization beginning with good trend monitoring documentation. Today, it is not technology, but the NDT expert and the inspector's judgment based on knowledge and experience, that is the prime ingredient of an effective internal test.

 

REFERENCES
Lorenz, Peter G., The Science of Remote Visual Inspection (RVI): Technology, Applications, Equipment, Lake Success, New York, Olympus, 1990.

United States Air Force, USAF 1951 Resolution Test Chart, USAF, 1951.

 

* 20 Division Ave., Massapequa, NY 11758; (516) 799-5968; fax (516) 799-5968; e-mail <lorenzpgl@aol.com>.

 

Copyright © 2003 by the American Society for Nondestructive Testing, Inc. All rights reserved.

 
 
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